Devices are in wide use today which receive and/or transmit information, including audio, video, data, etc., via radio frequency (RF) communication links. For example, cable transmission systems have become widely utilized to provide broadband communication of television signals, computer networking signals, voice over Internet protocol (VoIP) signals, and the like on a plurality of channels spread across a broad spectrum (e.g., 6 MHz or 8 MHz channels in the spectrum from 50 MHz to 1,000 MHz). Various devices, such as set-top-boxes, cable modems, television receivers, personal computer televisions (PCTV), and other customer premise equipment (CPE) devices have been employed for use with respect to such signals.
Such CPE devices often include a RF tuner (frequency converter), a demodulator, and an interface to other CPE devices, such as a network router, a network access point (AP), a personal computer (PC), a television set, etc. Such a RF tuner may be operated to select a subset of the channels out of the plurality of channels in the transmission spectrum from an input, such as an antenna input or cable input. The demodulator may then be operated to convert the single channel data from the tuner to a raw data output, usually baseband. The aforementioned interface, such as may comprise a PC interface or bridge, may operate to transport the raw data to a CPE, such as a PC, for the intended use.
Although the aforementioned channel selection has traditionally selected a single channel, more recently developed systems have been designed to select a plurality of channels, such as a block of contiguous channels, within the transmission spectrum. For example, version 3.0 of the Data Over Cable Service Interface Specification (DOCSIS) specifies tuners are to be sufficiently wideband to frequency convert no less than a 64 MHz block of spectrum and as much as a 96 MHz block of spectrum (i.e., provide no less than 64 MHz capture bandwidth and as much as 96 MHz capture bandwidth).
In operation of a RF tuner, an incoming signal at frequency fIN is mixed with a signal at frequency fLO1 from a local oscillator (LO) to produce a signal at a first intermediate frequency fIF1. In a dual conversion tuner configuration, this signal may then mixed with a signal at frequency fLO2 from a second local oscillator signal to produce a second intermediate frequency fIF2, such as for providing to a demodulator.
The foregoing frequency conversion process is illustrated in FIG. 1, wherein exemplary dual conversion tuner 100 is shown. In operation of tuner 100, the fLO signals (provided by LO 121 and LO 131) are mixed by respective mixers (mixer 120 and mixer 130) to frequency convert the input signal, fIN, to an intermediate frequency, fIF2, for providing to demodulator 140.
First IF filter 111 and second IF filter 112 provide filtering of the frequency converted signals output by the mixers. Accordingly, these filters ultimately determine capture bandwidth (CBW) of the tuner, here the bandwidth of fIF2. For example, the CBW of tuner 100 will be no broader than the convolution of filter 111 and filter 112). Of course, mixers 120 and 130 and other circuitry of tuner 100 must provide acceptable operation (e.g., linear operation, spurious signal suppression and/or avoidance, etc.) throughout a bandwidth consistent with the capture bandwidth defined by such filters for the tuner to actually provide the capture bandwidth.
An adverse effect of the dual conversion process of tuner 100 is the introduction of LO-related spurious signals, or “spurs,” into the tuned signal. These spurs are created by combinations of the harmonics of the LO frequencies used (fLO1 and fLO2). Because broader capture bandwidths result in more such spurious signals potentially falling within the capture bandwidth, as explained more fully below, traditional tuner circuits have been unable to provide capture bandwidths on the order of 64 MHz to 96 MHz as set forth in the DOCSIS 3.0 standard which otherwise meet signal quality requirements.
The frequency of each of the LO-related spurs can be calculated as:fSPUR=±nfLO1±mfLO2  (1)where n and m are integer numbers representing, respectively the harmonics of the high and low local oscillator frequencies, and f1 and f2 are the local oscillator frequencies (e.g., fLO1 and fLO2, respectively where fLO1>fLO2).
A spur generated by a given combination of fLO1 and fLO2 that falls within the capture bandwidth (CBW) of tuner 100 can degrade the quality of the output signal. In particular, a spur which is generated by multiples of fLO1 and fLO2 in a double conversion system will often have an amplitude (power level) which degrades the signal to noise and distortion (SINAD) ratio. Therefore, if such a spur falls within the capture bandwidth of a typical tuner circuit, its amplitude SN12 may corrupt the performance of the tuner itself.